Severe frequency selective fading is one of the major sources of signal degradation in terrestrial digital television (DTV) broadcasting. A cause of this frequency selective fading is multipath where multiple copies of the transmitted signal travel to the receiver through different paths. Multipath results in intersymbol interference (ISI) at the receiver, which creates data errors if no effective equalization is performed.
Multipath is more severe for mobile DTV where the receiver is installed in a vehicle moving at a high rate of speed. In this case, the wireless channel varies and becomes uncorrelated over a short period of time. Deep fades occur more frequently in such dynamic conditions than in the static conditions, challenging the design of robust equalizers for such mobile applications.
Over the past years, linear equalization techniques based on the zero-forcing (ZF) constraint or the minimum mean squared error (MMSE) criterion have been under extensive investigation. These techniques have been widely documented in the wireless communication literature, owing to their mathematical tractability and desirable simplicity. The dominant solution used for terrestrial DTV receivers, however, is the nonlinear decision-feedback equalizer (DFE).
The decision-feedback equalizer has been shown to have significant performance advantages over linear equalizers, yet without the discouraging complexity of optimum nonlinear equalizers. For practical realization, some form of updating process, such as the least mean square (LMS) and the recursive least squares (RLS) algorithms, is used to update decision-feedback equalizers for changing channel conditions. However, when there is a precipitous fade, the tracking loops of decision-feedback equalizers may lose lock, resulting in a convergence problem. If so, errors will propagate throughout the data stream until the equalizer is reinitialized. The consequence of this error propagation is serious for decision-feedback equalizer receivers due to their inherent error propagation deficiencies.
A promising solution to such problems is to employ antenna diversity at the receiver. Theoretically, as long as two antennas are separated by more than half a wavelength, the channel characteristics of the two paths are independent. Hence, the probability of concurrent deep fades is small. When one channel fades, accurate symbol decisions can still be obtained from the other channel, so that the update process can continue without divergence. Thus, system performance is significantly improved with joint operation of the multiple antennas.
In Japan, DTV systems based on orthogonal frequency division multiplexing (OFDM) have been claimed to be able to achieve accurate mobile reception using multiple receive antennas.
Antenna diversity in an equalizer, such as an MMSE-DFE equalizer, is proposed herein and can be used effectively for the mobile reception of television such as ATSC DTV signals based on eight-level vestigial sideband (8-VSB) modulation. A “double-threshold” switching scheme is further proposed in combination with the multiple antenna branches. Thus, weak antenna branches can be excluded from equalization and can be included, in a timely manner, when the receiver moves out of the fade condition. As a further addition or alternative, a “soft” combination of the active branches further improves system performance.